Anisotropy field and dynamic coercivity of L10 FePt nanoparticles close to the Curie temperature

An investigation of the intrinsic and dynamic coercivity of L1(0) FePt nanoparticles close to but below the Curie temperature is presented using Langevin dynamics simulations based on an atomistic spin and also a Landau-Lifshitz-Bloch (LLB) macrospin model. The anisotropy field in the critical temperature regime is best defined as the minimum reverse field along the easy axis resulting in switching by spontaneous thermal fluctuations of the magnetization. The temperature dependence of the anisotropy field for the classical spin model is then given by the relation H-K(T) similar or equal to H-K(0)(1-T/T-c)(1-beta), where beta = 0.366 is the critical exponent of the 3D Heisenberg model involved in the temperature dependence of the magnetization. The dynamic coercivity in the thermo-activated regime exhibits a logarithmic time dependence. No transition from elliptical to linear magnetic reversal is observed for 3 nm FePt nanoparticles, when the temperature is increased beyond the Curie point, which is a finite size effect.